6TiSCH T. Chang, Ed.
Internet-Draft M. Vucinic
Intended status: Standards Track Inria
Expires: October 10, 2019 X. Vilajosana
Universitat Oberta de Catalunya
S. Duquennoy
RISE SICS
D. Dujovne, Ed.
Universidad Diego Portales
April 8, 2019
6TiSCH Minimal Scheduling Function (MSF)
draft-ietf-6tisch-msf-03
Abstract
This specification defines the 6TiSCH Minimal Scheduling Function
(MSF). This Scheduling Function describes both the behavior of a
node when joining the network, and how the communication schedule is
managed in a distributed fashion. MSF builds upon the 6TiSCH
Operation Sublayer Protocol (6P) and the Minimal Security Framework
for 6TiSCH.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 10, 2019.
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Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Interface to the Minimal 6TiSCH Configuration . . . . . . . . 4
3. Autonomous Cells . . . . . . . . . . . . . . . . . . . . . . 4
4. Node Behavior at Boot . . . . . . . . . . . . . . . . . . . . 6
4.1. Start State . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Step 1 - Choosing Frequency . . . . . . . . . . . . . . . 6
4.3. Step 2 - Receiving EBs . . . . . . . . . . . . . . . . . 6
4.4. Step 3 - Setting up Autonomous Cells for the Join Process 7
4.5. Step 4 - Acquiring a RPL rank . . . . . . . . . . . . . . 7
4.6. Step 5 - Setting up Autonomous Cells for 6P . . . . . . . 7
4.7. Step 6 - Send EBs and DIOs . . . . . . . . . . . . . . . 8
4.8. End State . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Rules for Adding/Deleting Cells . . . . . . . . . . . . . . . 8
5.1. Adapting to Traffic . . . . . . . . . . . . . . . . . . . 8
5.2. Switching Parent . . . . . . . . . . . . . . . . . . . . 10
5.3. Handling Schedule Collisions . . . . . . . . . . . . . . 10
6. 6P SIGNAL command . . . . . . . . . . . . . . . . . . . . . . 11
7. Scheduling Function Identifier . . . . . . . . . . . . . . . 11
8. Rules for CellList . . . . . . . . . . . . . . . . . . . . . 12
9. 6P Timeout Value . . . . . . . . . . . . . . . . . . . . . . 12
10. Rule for Ordering Cells . . . . . . . . . . . . . . . . . . . 12
11. Meaning of the Metadata Field . . . . . . . . . . . . . . . . 12
12. 6P Error Handling . . . . . . . . . . . . . . . . . . . . . . 13
13. Schedule Inconsistency Handling . . . . . . . . . . . . . . . 13
14. MSF Constants . . . . . . . . . . . . . . . . . . . . . . . . 14
15. MSF Statistics . . . . . . . . . . . . . . . . . . . . . . . 14
16. Security Considerations . . . . . . . . . . . . . . . . . . . 14
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
17.1. MSF Scheduling Function Identifiers . . . . . . . . . . 15
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
18.1. Normative References . . . . . . . . . . . . . . . . . . 15
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18.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 16
Appendix B. Example of Implementation of SAX hash function . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
The 6TiSCH Minimal Scheduling Function (MSF), defined in this
specification, is a 6TiSCH Scheduling Function (SF). The role of an
SF is entirely defined in [RFC8480]. This specification complements
[RFC8480] by providing the rules of when to add/delete cells in the
communication schedule. This specification satisfies all the
requirements for an SF listed in Section 4.2 of [RFC8480].
MSF builds on top of the following specifications: the Minimal IPv6
over the TSCH Mode of IEEE 802.15.4e (6TiSCH) Configuration
[RFC8180], the 6TiSCH Operation Sublayer Protocol (6P) [RFC8480], and
the Minimal Security Framework for 6TiSCH
[I-D.ietf-6tisch-minimal-security].
MSF defines both the behavior of a node when joining the network, and
how the communication schedule is managed in a distributed fashion.
When a node running MSF boots up, it joins the network by following
the 7 steps described in Section 4. The end state of the join
process is that the node is synchronized to the network, has mutually
authenticated to the network, has identified a preferred routing
parent, and has scheduled one default managed cell (defined in
Section 5.1) to/from its preferred routing parent. After the join
process, the node can continuously add/delete/relocate cells, as
described in Section 5. It does so for 3 reasons: to match the link-
layer resources to the traffic, to handle changing parent, to handle
a schedule collision.
MSF is designed to operate in a wide range of application domains.
It is optimized for applications with regular upstream traffic (from
the nodes to the root).
This specification follows the recommended structure of an SF
specification, given in Appendix A of [RFC8480], with the following
adaptations:
o We have reordered some sections, in particular to have the section
on the node behavior at boot (Section 4) appear early in this
specification.
o We added sections on the interface to the minimal 6TiSCH
configuration (Section 2), the use of the SIGNAL command
(Section 6), the MSF constants (Section 14), the MSF statistics
(Section 15).
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o This specification does not include an examples section.
2. Interface to the Minimal 6TiSCH Configuration
A node implementing MSF SHOULD implement the Minimal 6TiSCH
Configuration [RFC8180], which defines the "minimal cell", a single
shared cell providing minimal connectivity between the nodes in the
network. The MSF implementation provided in this specification is
based on the implementation of the Minimal 6TiSCH Configuration.
However, an implementor MAY implements MSF without implementing
Minimal 6TiSCH Configuration.
MSF uses the minimal cell to exchange the following packets:
1. Enhanced Beacons (EBs), defined by [IEEE802154-2015]. These are
broadcast frames.
2. Boradcast DODAG Information Objects (DIOs), defined by [RFC6550].
Unicast DIOs SHOULD NOT be sent on minimal cell.
Because the minimal cell is SHARED, the back-off algorithm defined in
[IEEE802154-2015] is used to resolve collisions. To ensure there is
enough bandwidth available on the minimal cell, a node implementing
MSF SHOULD enforce some rules for limiting the traffic of broadcast
frames. For example, a Trickle Timer defined in [RFC6550] MAY be
applied on DIOs. However, this behvaior is implementation-specific
which is out of the scope of MSF.
As detailed in Section 2.2 of [RFC8480], MSF MUST schedule cells from
Slotframe 1, while Slotframe 0 is used for traffic defined in the
Minimal 6TiSCH Configuration. The length of Slotframe 0 and
Slotframe 1 SHOULD be the same value. The default of
SLOTFRAME_LENGTH is RECOMMENDED for both Slotframe 0 and Slotframe 1,
although any value can be advertised in the EBs.
3. Autonomous Cells
MSF nodes initialize Slotframe 1 with a set of default cells for
unicast communication with their neighbors. These cells are called
'autonomous cells', because they are maintained autonomously by each
node. Cells scheduled by 6P transaction are called 'managed cells'.
How to schedule managed cells is detailed in Section 5. For
autonomous cells, each node has:
o Autonomous Downstream Cell (AutoDownCell), one cell at a
[slotOffset,channelOffset] computed as a hash of its own EUI64
(detailed next). Its cell options bits are assigned as TX=1,
RX=1, SHARED=0.
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o Autonomous Upstream Cell (AutoUpCell), one cell at a
[slotOffset,channelOffset] computed as a hash of the EUI64 of the
Join Proxy or its RPL routing parent (detailed in Section 4.4).
Its cell options bits are assigned as TX=1, RX=1, SHARED=1.
To compute a [slotOffset,channelOffset] from an EUI64 address, nodes
MUST use the hash function SAX [SAX-DASFAA]. The coordinates are
computed to distribute the cells across all channel offsets, and all
but the first time offsets of Slotframe 1. The first time offset is
skipped to avoid colliding with the minimal cell in Slotframe 0. The
slot coordinates derived from a given EUI64 address are computed as
follows:
o slotOffset(MAC) = 1 + hash(EUI64, length(Slotframe_1) - 1)
o channelOffset(MAC) = hash(EUI64, 16)
The second input parameter defines the maxmium return value of the
hash function. There are other optional parameters defined in SAX
determines the performance of SAX hash function. Those parameters
could be broadcasted in EB frame or pre-configured. For
interoperability purposes, an example how the hash function is
implemented is detailed in Appendix B.
Because of hash collisions, there will be cases when the AutoUpCell
and AutoDownCell are scheduled at the same slot offset and/or channel
offset. Hash collisions among a set of cells at a given time offset
is resolved at run-time as follows:
o The AutoUpCell with the most packets in the outgoing queue takes
precedence.
o If the AutoUpCell have empty outgoing queues, the AutoDownCell
takes precedence.
In case the autonomous cell to be installed is conflicted with a
managed cell, a 6P RELOCATE command MUST be issued to the responding
neighbor to relocate the conflicting managed cell.
The traffic on the autonomous cells are scheduled as:
o Join Request packets and 6P ADD/DELETE Request frames to the
node's Join Proxy or its RPL routing parent MUST be sent on
AutoUpCell.
o Join Response packets and 6P ADD/DELETE Response frames to the
pledge or its RPL routing child MUST be sent on AutoDownCell.
o 6P RELOCATE Request frames to the node's RPL routing child MUST be
sent on AutoDownCell.
o 6P RELOCATE Response frames to its RPL routing parent MUST be sent
on AutoUpCell.
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Throughout the network lifetime, nodes MUST maintain the autonomous
cells as follows:
o The AutoDownCell MUST always remain scheduled.
o When a new Join Proxy (JP) is selected, add an AutoUpCell to it.
o When the node is secured after joining process, remove the
AutoUpCell to the JP.
o When a new RPL routing parent is selected, add an AutoUpCell to
it.
o When the routing parent is de-selected, remove the AutoUpCell to
it.
o 6P CLEAR MUST NOT erase any autonomous cells.
4. Node Behavior at Boot
This section details the behavior the node SHOULD follow from the
moment it is switched on, until it has successfully joined the
network. Section 4.1 details the start state; Section 4.8 details
the end state. The other sections detail the 6 steps of the joining
process. We use the term "pledge" and "joined node", as defined in
[I-D.ietf-6tisch-minimal-security].
4.1. Start State
A node implementing MSF MUST implement the Minimal Security Framework
for 6TiSCH [I-D.ietf-6tisch-minimal-security]. As a corollary, this
means that a pledge, before being switched on, may be pre-configured
with the Pre-Shared Key (PSK) for joining, as well as any other
configuration detailed in ([I-D.ietf-6tisch-minimal-security]). This
is not needed if the node implements a security solution not based on
PSKs, such as ([I-D.ietf-6tisch-dtsecurity-zerotouch-join]).
4.2. Step 1 - Choosing Frequency
When switched on, the pledge SHOULD randomly choose a frequency among
the available frequencies, and start listening for EBs on that
frequency.
4.3. Step 2 - Receiving EBs
Upon receiving the first EB, the pledge SHOULD continue listening for
additional EBs to learn:
1. the number of neighbors N in its vicinity
2. which neighbor to choose as a Join Proxy (JP) for the joining
process
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While the exact behavior is implementation-specific, the RECOMMENDED
behavior is to follow [RFC8180], and listen until EBs sent by
NUM_NEIGHBOURS_TO_WAIT nodes (defined in [RFC8180]) have been
received.
During this step, the pledge MAY synchronize to any EB it receives
from the network it wishes to join. How to decide whether an EB
originates from a node from the network it wishes to join is
implementation-specific, but MAY involve filtering EBs by the PAN ID
field it contains, the presence and contents of the IE defined in
[I-D.richardson-6tisch-join-enhanced-beacon], or the key used to
authenticate it.
The decision of which neighbor to use as a JP is implementation-
specific, and discussed in [I-D.ietf-6tisch-minimal-security].
4.4. Step 3 - Setting up Autonomous Cells for the Join Process
After selected a JP, a node MUST set up an AutoUpCell to that JP, as
described in Section 3. A Join Request is then sent then by the
pledge to its JP over the AutoUpCell. The JP forwards the Join
Request to the JRC, possibly over multiple hops, over the AutoUpCells
as well. Similarly, the JRC sends the Join Response to the JP,
possibly over multiple hops, over the AutoDownCells. When JP
received the Join Response from the JRC, it sends that Join Response
to the pledge over its AutoDownCell. The pledge thereby learns the
keying material used in the network, as well as other configurations,
and becomes a "joined node". The AutoUpCell to the JP is removed at
the same time by the "joined node".
4.5. Step 4 - Acquiring a RPL rank
Per [RFC6550], the joined node receives DIOs, computes its own rank,
and selects a preferred parent.
4.6. Step 5 - Setting up Autonomous Cells for 6P
After selected a preferred parent, the joined node MUST set up an
AutoUpCell to that parent. Then it MUST issue a 6P ADD command MUST
to that parent, with the following fields:
o CellOptions: set to TX=1,RX=0,SHARED=0
o NumCells: set to 1
o CellList: at least 5 cells, chosen according to Section Section 8
In case the 6P ADD transaction failed, the node MUST issue another 6P
ADD command and repeat until the one cell is installed to the parent.
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4.7. Step 6 - Send EBs and DIOs
The node SHOULD start sending EBs and DIOs on the minimal cell, while
following the transmit rules for broadcast frames from Section 2.
4.8. End State
For a new node, the end state of the joining process is:
o it is synchronized to the network
o it is using the link-layer keying material it learned through the
secure joining process
o it has identified its preferred routing parent
o it has one AutoUpCell to its parent and one AutoDownCell
o it has one Managed Tx Cell to its parent
o it starts to send DIOs, potentially serving as a router for other
nodes' traffic
o it starts to send EBs, potentially serving as a JP for new pledge
5. Rules for Adding/Deleting Cells
Once a node has joined the 6TiSCH network, it adds/deletes/relocates
cells with its preferred parent for three reasons:
o to match the link-layer resources to the traffic between the node
and its preferred parent (Section 5.1)
o to handle switching preferred parent or(Section 5.2)
o to handle a schedule collision (Section 5.3)
5.1. Adapting to Traffic
A node implementing MSF MUST implement the behavior described in this
section.
The goal of MSF is to manage the communication schedule in the 6TiSCH
schedule in a distributed manner. For a node, this translates into
monitoring the current usage of the cells it has to its preferred
parent:
o If the node determines that the number of link-layer frames it is
attempting to exchange with its preferred parent per unit of time
is larger than the capacity offered by the TSCH managed cells it
has scheduled with it, the node issues a 6P ADD command to its
preferred parent to add one managed Tx cell to the TSCH schedule.
o If the traffic is lower than the capacity, the node issues a 6P
DELETE command to its preferred parent to delete one managed cell
from the TSCH schedule.
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Adding/removing/relocating cells involves exchanging frames that
contain 6P commands. All 6P Request frames to its parent MUST be
sent on the AutoUpCell All 6P Response frames to non-parent neighbor
MUST be sent on AutoDownCell. In case a managed cell from non-parent
is conflicted with AutoUpCell to be installed, a 6P RELOCATE command
needs to be issued to the neighbor, as mentioned in Section 3. The
6P RELOCATE Request frame MUST be sent on AutoDownCell and the
Response MUST be sent on AutoUpCell.
The node MUST maintain the following counters for its preferred
parent:
NumCellsElapsed : Counts the number of managed cells that have
elapsed since the counter was initialized. This counter is
initialized at 0. Each time the TSCH state machine indicates
that the current cell is a managed cell to the preferred parent,
NumCellsElapsed is incremented by exactly 1, regardless of
whether the cell is used to transmit/receive a frame.
NumCellsUsed: Counts the number of managed cells that have been
used. This counter is initialized at 0. NumCellsUsed is
incremented by exactly 1 when, during a managed cell to the
preferred parent, either of the following happens:
* The node sends a frame to its preferred parent. The counter
increments regardless of whether a link-layer acknowledgment
was received or not.
* The node receives a frame from its preferred parent.
Both NumCellsElapsed and NumCellsUsed counters can be used to cell
with cell option TX=1 or RX=1. All the frames used for increasing/
decreasing the counters MUST be encrypted or decryptable with the key
get from joining process.
Implementors MAY choose to create the same counters for each
neighbor, and add them as additional statistics in the neighbor
table.
The counters are used as follows:
1. Both NumCellsElapsed and NumCellsUsed are initialized to 0 when
the node boots.
2. When the value of NumCellsElapsed reaches MAX_NUMCELLS:
* If NumCellsUsed > LIM_NUMCELLSUSED_HIGH, trigger 6P to add a
single cell to the preferred parent
* If NumCellsUsed < LIM_NUMCELLSUSED_LOW, trigger 6P to remove a
single cell to the preferred parent
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* Reset both NumCellsElapsed and NumCellsUsed to 0 and go to
step 2.
5.2. Switching Parent
A node implementing MSF SHOULD implement the behavior described in
this section.
Part of its normal operation, the RPL routing protocol can have a
node switch preferred parent. The procedure for switching from the
old preferred parent to the new preferred parent is:
1. if there is managed cell conflicted with the AutoUpCells to be
installed, the node MUST issue a 6P RELOCATE command to relocate
the conflicted cell
2. if there is no conflicted cell, the node installs the AutoUpCells
to its new parent
3. the node counts the number of managed cells it has per slotframe
to the old preferred parent
4. the node triggers one or more 6P ADD commands to schedule the
same number of managed cells to the new preferred parent
5. when that successfully completes, the node issues a 6P CLEAR
command to its old preferred parent
5.3. Handling Schedule Collisions
A node implementing MSF SHOULD implement the behavior described in
this section. The "MUST" statements in this section hence only apply
if the node implements schedule collision handling.
Since scheduling is entirely distributed, there is a non-zero
probability that two pairs of nearby neighbor nodes schedule a
managed cell at the same [slotOffset,channelOffset] location in the
TSCH schedule. In that case, data exchanged by the two pairs may
collide on that cell. We call this case a "schedule collision".
The node MUST maintain the following counters for each managed
unicast cell to its preferred parent:
NumTx: Counts the number of transmission attempts on that cell.
Each time the node attempts to transmit a frame on that cell,
NumTx is incremented by exactly 1.
NumTxAck: Counts the number of successful transmission attempts on
that cell. Each time the node receives an acknowledgment for a
transmission attempt, NumTxAck is incremented by exactly 1.
Implementors MAY choose to maintain the same counters for each
managed cell in the schedule.
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Since both NumTx and NumTxAck are initialized to 0, we necessarily
have NumTxAck <= NumTx. We call Packet Delivery Ratio (PDR) the
ratio NumTxAck/NumTx; and represent it as a percentage. A cell with
PDR=50% means that half of the frames transmitted are not
acknowledged (and need to be retransmitted).
Each time the node switches preferred parent (or during the join
process when the node selects a preferred parent for the first time),
both NumTx and NumTxAck MUST be reset to 0. They increment over
time, as the schedule is executed and the node sends frames to its
preferred parent. When NumTx reaches 256, both NumTx and NumTxAck
MUST be divided by 2. That is, for example, from NumTx=256 and
NumTxAck=128, they become NumTx=128 and NumTxAck=64. This operation
does not change the value of the PDR, but allows the counters to keep
incrementing.
The key for detecting a schedule collision is that, if a node has
several cells to the same preferred parent, all cells should exhibit
the same PDR. A cell which exhibits a PDR significantly lower than
the others indicates than there are collisions on that cell.
Every HOUSEKEEPINGCOLLISION_PERIOD, the node executes the following
steps:
1. It computes, for each managed unicast cell with its preferred
parent (not for the autonomous cell), that cell's PDR.
2. Any cell that hasn't yet had NumTx divided by 2 since it was last
reset is skipped in steps 3 and 4. This avoids triggering cell
relocation when the values of NumTx and NumTxAck are not
statistically significant yet.
3. It identifies the cell with the highest PDR.
4. For any other cell, it compares its PDR against that of the cell
with the highest PDR. If the different is less than
RELOCATE_PDRTHRES, it triggers the relocation of that cell using
a 6P RELOCATE command.
6. 6P SIGNAL command
The 6P SIGNAL command is not used by MSF.
7. Scheduling Function Identifier
The Scheduling Function Identifier (SFID) of MSF is
IANA_6TISCH_SFID_MSF.
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8. Rules for CellList
MSF uses 2-step 6P Transactions exclusively. 6P Transactions are
only initiated by a node towards it preferred parent. As a result,
the cells to put in the CellList of a 6P ADD command, and in the
candidate CellList of a RELOCATE command, are chosen by the node
initiating the 6P Transaction. In both cases, the same rules apply:
o The CellList SHOULD contain 5 or more cells.
o Each cell in the CellList MUST have a different slotOffset value.
o For each cell in the CellList, the node MUST NOT have any
scheduled cell on the same slotOffset.
o The slotOffset value of any cell in the CellList MUST NOT be the
same as the slotOffset of the minimal cell (slotOffset=0).
o The slotOffset of a cell in the CellList SHOULD be randomly and
uniformly chosen among all the slotOffset values that satisfy the
restrictions above.
o The channelOffset of a cell in the CellList SHOULD be randomly and
uniformly chosen in [0..numFrequencies], where numFrequencies
represents the number of frequencies a node can communicate on.
9. 6P Timeout Value
It is calculated for the worst case that a 6P response is received,
which means the 6P response is sent out successfully at the very
latest retransmission. And for each retransmission, it backs-off
with largest value. Hence the 6P timeout value is calcualted as
((2^MAXBE)-1)*SLOTFRAME_LENGTH, where:
o MAXEB is the maxmium backoff exponent is used
o SLOTFRAME_LENGTH represents the length of slotframe
10. Rule for Ordering Cells
Cells are ordered slotOffset first, channelOffset second.
The following sequence is correctly ordered (each element represents
the [slottOffset,channelOffset] of a cell in the schedule):
[1,3],[1,4],[2,0],[5,3],[6,0],[6,3],[7,9]
11. Meaning of the Metadata Field
The Metadata field is not used by MSF.
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12. 6P Error Handling
Section 6.2.4 of [RFC8480] lists the 6P Return Codes. Figure 1 lists
the same error codes, and the behavior a node implementing MSF SHOULD
follow.
+-----------------+----------------------+
| Code | RECOMMENDED behavior |
+-----------------+----------------------+
| RC_SUCCESS | nothing |
| RC_EOL | nothing |
| RC_ERR | quarantine |
| RC_RESET | quarantine |
| RC_ERR_VERSION | quarantine |
| RC_ERR_SFID | quarantine |
| RC_ERR_SEQNUM | clear |
| RC_ERR_CELLLIST | clear |
| RC_ERR_BUSY | waitretry |
| RC_ERR_LOCKED | waitretry |
+-----------------+----------------------+
Figure 1: Recommended behavior for each 6P Error Code.
The meaning of each behavior from Figure 1 is:
nothing: Indicates that this Return Code is not an error. No error
handling behavior is triggered.
clear: Abort the 6P Transaction. Issue a 6P CLEAR command to that
neighbor (this command may fail at the link layer). Remove all
cells scheduled with that neighbor from the local schedule. Keep
that node in the neighbor and routing tables.
quarantine: Same behavior as for "clear". In addition, remove the
node from the neighbor and routing tables. Place the node's
identifier in a quarantine list for QUARANTINE_DURATION. When in
quarantine, drop all frames received from that node.
waitretry: Abort the 6P Transaction. Wait for a duration randomly
and uniformly chosen in [WAITDURATION_MIN,WAITDURATION_MAX].
Retry the same transaction.
13. Schedule Inconsistency Handling
The behavior when schedule inconsistency is detected is explained in
Figure 1, for 6P Return Code RC_ERR_SEQNUM.
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14. MSF Constants
Figure 2 lists MSF Constants and their RECOMMENDED values.
+------------------------------+-------------------+
| Name | RECOMMENDED value |
+------------------------------+-------------------+
| KA_PERIOD | 10 s |
| LIM_NUMCELLSUSED_HIGH | 75 % |
| LIM_NUMCELLSUSED_LOW | 25 % |
| HOUSEKEEPINGCOLLISION_PERIOD | 1 min |
| RELOCATE_PDRTHRES | 50 % |
| SLOTFRAME_LENGTH | 101 slots |
| QUARANTINE_DURATION | 5 min |
| WAITDURATION_MIN | 30 s |
| WAITDURATION_MAX | 60 s |
+------------------------------+-------------------+
Figure 2: MSF Constants and their RECOMMENDED values.
15. MSF Statistics
Figure 3 lists MSF Statistics and their RECOMMENDED width.
+-----------------+-------------------+
| Name | RECOMMENDED width |
+-----------------+-------------------+
| NumCellsElapsed | 1 byte |
| NumCellsUsed | 1 byte |
| NumTx | 1 byte |
| NumTxAck | 1 byte |
+-----------------+-------------------+
Figure 3: MSF Statistics and their RECOMMENDED width.
16. Security Considerations
MSF defines a series of "rules" for the node to follow. It triggers
several actions, that are carried out by the protocols defined in the
following specifications: the Minimal IPv6 over the TSCH Mode of IEEE
802.15.4e (6TiSCH) Configuration [RFC8180], the 6TiSCH Operation
Sublayer Protocol (6P) [RFC8480], and the Minimal Security Framework
for 6TiSCH [I-D.ietf-6tisch-minimal-security]. In particular, MSF
does not define a new protocol or packet format.
MSF relies entirely on the security mechanisms defined in the
specifications listed above.
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17. IANA Considerations
17.1. MSF Scheduling Function Identifiers
This document adds the following number to the "6P Scheduling
Function Identifiers" sub-registry, part of the "IPv6 over the TSCH
mode of IEEE 802.15.4e (6TiSCH) parameters" registry, as defined by
[RFC8480]:
+----------------------+-----------------------------+-------------+
| SFID | Name | Reference |
+----------------------+-----------------------------+-------------+
| IANA_6TISCH_SFID_MSF | Minimal Scheduling Function | RFCXXXX |
| | (MSF) | (NOTE:this) |
+----------------------+-----------------------------+-------------+
Figure 4: IETF IE Subtype '6P'.
18. References
18.1. Normative References
[I-D.ietf-6tisch-dtsecurity-zerotouch-join]
Richardson, M., "6tisch Zero-Touch Secure Join protocol",
draft-ietf-6tisch-dtsecurity-zerotouch-join-03 (work in
progress), October 2018.
[I-D.ietf-6tisch-minimal-security]
Vucinic, M., Simon, J., Pister, K., and M. Richardson,
"Minimal Security Framework for 6TiSCH", draft-ietf-
6tisch-minimal-security-10 (work in progress), April 2019.
[I-D.richardson-6tisch-join-enhanced-beacon]
Dujovne, D. and M. Richardson, "IEEE802.15.4 Informational
Element encapsulation of 6tisch Join Information", draft-
richardson-6tisch-join-enhanced-beacon-03 (work in
progress), January 2018.
[IEEE802154-2015]
IEEE standard for Information Technology, "IEEE Std
802.15.4-2015 Standard for Low-Rate Wireless Personal Area
Networks (WPANs)", December 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
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[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
.
[RFC8180] Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
May 2017, .
[RFC8480] Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH
Operation Sublayer (6top) Protocol (6P)", RFC 8480,
DOI 10.17487/RFC8480, November 2018,
.
18.2. Informative References
[SAX-DASFAA]
Ramakrishna, M. and J. Zobel, "Performance in Practice of
String Hashing Functions", DASFAA , 1997.
Appendix A. Contributors
Beshr Al Nahas (Chalmers University, beshr@chalmers.se) Olaf
Landsiedel (Chalmers University, olafl@chalmers.se) Yasuyuki Tanaka
(Inria-Paris, yasuyuki.tanaka@inria.fr)
Appendix B. Example of Implementation of SAX hash function
For the consideration of interoperability, this section provides an
example of implemention SAX hash function [SAX-DASFAA]. The input
parameters of the function are:
o T, which is the hashing table length
o c, which is the characters of string s, to be hashed
In MSF, the T is replaced by the length slotframe 1. String s is
replaced by the mote EUI64 address. The characters of the string c0,
c1, ..., c7 are the 8 bytes of EUI64 address.
The SAX hash function requires shift operation which is defined as
follow:
o L_shift(v,b), which refers to left shift variable v by b bits
o R_shift(v,b), which refers to right shift variable v by b bits
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The steps to calculate the hash value of SAX hash function are:
1. initialize variable h to h0 and variable i to 0, where h is the
intermediate hash value and i is the index of the bytes of EUI64
address
2. sum the value of L_shift(h,l_bit), R_shift(h,r_bit) and ci
3. calculate the result of exclusive or bewteen the sum value in
Step 2 and h
4. modulo the result of Step 3 by T
5. assign the result of Step 4 to h
6. increase i by 1
7. repeat Step2 to Step 6 until i reaches to 8
8. assign the result of Step 5 to h
The value of variable h the hash value of SAX hash function.
For interoperability purposes, the values of h0, l_bit and r_bit in
Step 1 and 2 are configured as:
o h0 = 0
o l_bit = 0
o r_bit = 1
The appropriate values of l_bit and r_bit could vary depending on the
the set of motes' EUI64 address. How to find those values is out of
the scope of this specification.
Authors' Addresses
Tengfei Chang (editor)
Inria
2 rue Simone Iff
Paris 75012
France
Email: tengfei.chang@inria.fr
Malisa Vucinic
Inria
2 rue Simone Iff
Paris 75012
France
Email: malisa.vucinic@inria.fr
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Xavier Vilajosana
Universitat Oberta de Catalunya
156 Rambla Poblenou
Barcelona, Catalonia 08018
Spain
Email: xvilajosana@uoc.edu
Simon Duquennoy
RISE SICS
Isafjordsgatan 22
164 29 Kista
Sweden
Email: simon.duquennoy@ri.se
Diego Dujovne (editor)
Universidad Diego Portales
Escuela de Informatica y Telecomunicaciones
Av. Ejercito 441
Santiago, Region Metropolitana
Chile
Phone: +56 (2) 676-8121
Email: diego.dujovne@mail.udp.cl
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